Column filter using bundles of long fibers

ABSTRACT

A column filter using bundles of long fibers is disclosed. The bundles of long fibers are fixed at their lower end portions but are free-standing at their upper end portions. The length of the fibers in the bundles may preferably be longer than 1,000 mm but shorter than 3,000 mm--especially longer than 2,000 mm but shorter than 3,000 mm. During operation, the bundles may be compressed to a height of 80-30%, generally 50-40% of their original height. A plurality of holders for the respective bundles of long fibers, each of said holders being formed of a cap and a tube communicating to each other, said cap defining orifices and said tube defining a slot through a side wall and opening at both upper and lower ends, may preferably be provided on a perforated plate arranged transversely in a lower interior part of the shell with the cap being located above the perforated plate and the tube being positioned below the perforated plate. The holders cover substantially all perforations of the perforated plate. The lower end portion of each of the bundles is fixed to a lower periphery of a side wall of the associated cap.

BACKGROUND OF THE INVENTION

(a) Field of the Invention:

This invention relates to a column filter which uses bundles of longfibers as a filter medium to remove at a high rate suspended solids fromraw liquids such as city water, industrial water, sewage, river water,lake or pond water, supernatant waters from coagulation and settlingtreatments, waters discharged intermediately during the practice ofvarious processes, recovered waters such as those from pulp-andpaper-making processes, various waste waters, processing waters orvaluable-material-containing liquids from biological treatmentapparatus, alcoholic beverages, oils and the like. In particular, thisinvention is concerned with a column filter of the above sort, in whichan improvement has been applied to the structure holding lower endportions of the bundles of long fibers.

(b) Description of the Related Art:

A variety of filters have conventionally been used to remove suspendedsolids from a raw liquid.

They may be classified into several different types, including filtersadapted for the removal of relatively large suspended solids from a rawliquid by a screen such as a wire mesh, filters employed for themicrofiltration of fine particles from a raw liquid by a filter membranehaving micropores, and filters using filter elements, which carry aprecoat layer of a filter aid coated thereon, for the removal ofsuspended solids.

Various filters are thus used depending on the kinds of raw liquids andthe purposes of their filtration. Among these, filters of the packedmedium type are most common.

A typical packed medium filter has a cylindrical upright shell with apacked layer of a granular filter medium such as filter sand oranthracite or a fibrous filter medium such as short fibers or fiberballs. A raw liquid is passed as a downflow or upflow through the packedmedium layer so that solids suspended in the raw liquid may be trappedin the packed layer to obtain a filtered liquid, namely, filtrate.Filtration is stopped when the pressure drop has increased to apredetermined level or the quality of the filtered liquid has fallen toa prescribed level. The packed medium layer is then flushed with wateror expanded or agitated by a water stream, and air or the like, so thatthe solids thus trapped may be flushed away to permit resumption of thefiltration through the packed medium layer thus flushed.

With packed medium filters it is desired that in the filtration step thepressure drop due to the filter medium resistance does not increasesubstantially and that the quality of the filtered liquid remains goodeven when filtration is at a high flow rate; and that in the washingstep the solids trapped in the packed medium layer can be flushedcompletely with a minimum of both water and time.

A filter using a conventional granular filter medium such as sand oranthracite is however accompanied by drawbacks--feed velocity of theliquid through the filter medium can be as low as 20 m/hr even at themaximum, and, depending on the kind of the suspended solids, the filtermedium is prone to prematured clogging and results in an increasedpressure drop at a relatively early stage. During washing, this filterdevelops further drawbacks--washing must be at a flow rate slow enoughto prevent the granular filter medium from flowing out of the shell, andthe washing thus tends to be insufficient to prevent the gradualaccumulation of suspended solids and, in some instances, the formationof mud balls or the like may result. There are still further drawbacksin that a relatively large quantity of washing water is required and thelevel of concentration of suspended solids in the effluent of washing issmall.

A filter using a medium such as short fibers or fibrous balls allowsfree choice in size of the filter medium. It thus has an advantage thatan optimal filter medium can be chosen depending on the kind of solidssuspended in a raw liquid. Because the filter medium used has anextremely light weight, the filter is accompanied by drawbacks; forexample, a screen or the like must be provided to prevent the filtermedium from flowing out of the shell during the washing, the structureof the filter tends to become complex, and the screen becomes anobstacle to hamper the removal of trapped solids from the shell duringwashing, so that the washing cannot be effected sufficiently, resultingin the gradual accumulation of trapped solids.

A high rate filter--which solves the abovedescribed drawbacks ofconventional packed medium filters, permits filtration at a high ratewithout any substantial increase in the pressure drop by filtrationresistance, and can be washed completely with a small quantity ofwashing water in a short time, by using a filter with bundles of longfibers--has already been proposed by the present inventors in JapanesePatent Application No. 146989/1987.

The above-proposed high rate filter has a support provided transverselyor packed within a shell and bundles of long fibers 400-1,000 mm longfixed at lower end portions thereof on an upper part of the support andfree-standing at upper end portions thereof, whereby a filter medium isformed. A raw liquid is passed as a downflow from the upper end portionsof the fibers toward the lower end portions thereof, so that suspendedsolids may be trapped in interstitial spaces among the long fibers.

The use of a bundled long fiber filter has various advantagesunavailable from the use of filters packed with conventional filter sandor anthracite; for example, a high filtration rate, pressure drop doesnot increase to any substantial extent, and complete washing is possiblewith a smaller quantity of wash water and in a shorter time. However,the filter has the problem that the operable filtration time is rathershort, which is not satisfactory.

SUMMARY OF THE INVENTION

An object of this invention is to solve the above-described drawbacks ofthe filter proposed previously, which uses bundles of long fibers as afilter medium, and hence to provide a column filter featuring a longeroperable filtration time while retaining the advantages of the filterproposed previously.

Another object of this invention is to provide a column filter capableof exhibiting the advantages of bundles of long fibers to a maximumextent, so that the bundles of long fibers can be washed thoroughly bybackwashing and leakage of suspended solids into a filtered liquid canbe minimized.

In one aspect of this invention, there is thus provided a column filterusing bundles of long fibers. The filter is provided with an uprightcylindrical shell and the bundles of long fibers (3) being arrangedsubstantially upright inside the shell with lower end portions thereofbeing fixed and upper end portions thereof being free-standing, whereby,when a raw liquid with suspended solids therein is fed to a top part ofthe shell and is then allowed to pass from the upper end portions of thebundles of long fibers toward their lower end portions, the suspendedsolids are at least partly trapped in interstices among the fibers. Asupporting member is arranged transversely in a lower interior part ofthe shell, the bundles of long fibers have a length in a range longerthan 1,000 mm but shorter than 3,000 mm and are fixed at the lower endportions thereof on an upper surface of said supporting member. Thepacking density of the bundles is in a range such that the bundles mayretain 30-80% of the original height thereof while the raw liquid withsuspended solids is being fed into the shell and compressing the bundlestoward the supporting member. The packing density of the bundles maypreferably be in a range of 25-110 kg in terms of their dry weight perbulk volume m³ of the bundles. The fibers in the bundles may preferablyhave a diameter up to 80 μm.

In another aspect of this invention, there is also provided a columnfilter using bundles of long fibers. The filter is provided with anupright cylindrical shell and the bundles of long fibers being arrangedsubstantially upright inside the shell with lower end portions thereofbeing fixed and upper end portions thereof being free-standing, whereby,when a raw liquid with suspended solids therein is fed to a top part ofthe shell and is then allowed to pass from the upper end portions of thebundles of long fibers toward their lower end portions, the suspendedsolids are at least partly trapped in interstices among the fibers. Aplurality of holders for the respective bundles of long fiber, each ofsaid holders being formed of an upper cap member and a lower hollowmember communicating to reach other, said upper cap member definingopenings therethrogh and said lower hollow member defining anaxially-elongated opening through a side wall thereof and opening atboth upper and lower ends thereof, are provided on a perforated platearranged transversely in a lower interior part of the shell with theupper cap member being located above the perforated plate and the lowerhollow member being positioned below the perforated plate. The holderscover substantially all perforations of the perforated plate. The lowerend portion of each of the bundles of long fibers is fixed to a lowerperiphery of a side wall of the associated upper cap member. The lengthof the long fiber bundles may preferably be longer than 1,000 mm butshorter than 3,000 mm--especially longer than 2,000 mm but shorter than3,000 mm.

As the shape of each upper cap member, a cylindrical,polygonally-cylindrical, truncated conical, polygonally-conicaltruncated or hemispherical shape may be chosen suitably. Similarly, thelower hollow member may be formed into a cylindrical,polygonally-cylindrical, truncated conical, polygonally-conicaltruncated or hemispherical shape as desired.

The fixing of the lower end portions of the bundles of long fibers canbe effected, for example, by pinching the lower end portions, adheringthe lower end portions, or fusion-bonding the lower end portionsthemselves.

In the column filter according to this invention, the bundles of longfibers are not merely provided at the individual perforations of theperforated plate. The plurality of holders for the respective bundles oflong fibers, each of said holders being formed of the upper cap memberand the lower hollow member communicating to each other, said upper capmember defining the openings therethrough and said lower hollow memberdefining the axially-elongated opening through the side wall thereof andopening at both upper and lower ends thereof, are provided on theperforated plate arranged transversely in the lower part of the shellwith the upper cap member being located above the perforated plate andthe lower hollow member being positioned below the perforated plate. Theholders cover substantially all perforations of the perforated plate.The lower end portion of each of bundles of long fibers is fixed to thelower periphery of the side wall of the associated upper cap member. Theopenings of each upper cap member are therefore surrounded substantiallyby the associated bundles of long fibers. Suspended solids are hencehardly allowed to leak into a filtered liquid. Upon backwashing, a fluidsuch as compressed air or back-washing water is allowed to flow outevenly through the individual openings of each upper cap member, so thatdense parts of the bundles of long fibers, said parts being located inthe proximity of the perforated plate, can be effectively washed toensure sufficient backwashing.

The column filter according to this invention allows to performbackwashing with backwashing water and/or compressed air of a high flowvelocity in a short time. Compared to the conventional filters, thebackwashing time can be shortened significantly and trapped solids canbe surely discharged from the shell by backwashing. Different from theconventional filters, the column filter according to this invention isfree from the accumulation of trapped solids and requires backwashingwater in a far smaller quantity.

Further, the column filter according to this invention allows toincrease the flow velocity to a considerable extent compared to theconventional filters. It is hence possible to reduce the crosssectionalarea of the column filter. This obviously leads to a considerablereduction in installation cost and space.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 through 3 illustrate a column filter according to a firstembodiment of this invention, in which:

FIG. 1 is a simplified vertical cross-section of the column filter in aninitial stage of filtration,

FIG. 2 is a simplified vertical cross-section of the column filter in afinal stage of filtration, and

FIG. 3 is a simplified vertical cross-section of the column filter underbackwashing;

FIG. 4 is a vertical cross-section showing the overall construction ofthe column filter according to the first embodiment of this invention,in which some modifications have been incorporated;

FIG. 5 is an enlarged vertical cross-section of each holder for bundlesof long fibers, said holder being used in the column filter of FIG. 4;

FIG. 6 is a partly cut-away perspective view of the holder of FIG. 5;

FIG. 7 is similar to FIG. 5 but shows a modified holder;

FIG. 8 is a flow diagram of a conventional column filter using bundlesof long fibers; and

FIG. 9 is an enlarged fragmentary vertical cross-section illustratingthe construction of a lower part of the conventional column filter ofFIG. 8.

DETAILED DESCRIPTION OF THE INVENTION

Taking water with solids suspended therein as an exemplary raw liquid,the present invention will hereinafter be described in detail withreference to the drawings.

Referring first to FIG. 1, a perforated plate 2 is arranged transverselyin a lower interior part of an upright cylindrical shell 1. Bundles offibers 3, which are longer than 1,000 mm but shorter than 3,000 mm, arefixed at lower end portions thereof on the plate 2, while their upperend portions are free-standing. A raw water feed line 4 is provided incommunication with a top part of the shell 1 and a backwash waterdischarge line 5 is connected as a branch to the raw water feed line 4.

Further, a filtered water outlet line 6 is provided in communicationwith a bottom part of the shell 1. Backwash water feed line 7 and airfeed line 8 are also provided as branches in communication with theoutlet line 6. Designated at numerals 9, 10, 11, 12 and 13 are valves.

Preferably, the bundles of long fibers 3 are packed in such a quantitythat even when a raw water is passed as a downflow at a relatively highflow rate, the bundled long fibers 3 may be slightly bent and compressedto a height from 80% to 30% of their original height. However, thebundled long fibers 3 should not be bent horizontally by the stream ofthe raw water and should remain substantially upright as a whole insidethe shell 1. In other words, the height reduction of the bundled longfibers 3 ranges from 20% to 70%, generally about 40-50% while the rawliquid with suspended solids is being fed. This packing quantity canprovide a filter member having an extremely large interstice volume.

Filtration of the raw water through the filter according to the firstembodiment of this invention can be conducted in the following manner.

The valves 9,13 are opened, and the raw water containing suspendedsolids is fed as a downflow by way of the raw water feed line 4.

By the stream of the raw water, the bundled long fibers 3 are somewhatbent at lower portions thereof and are reduced in height at an initialstage as shown in FIG. 1. The bundled long fibers 3 however remainupright as a whole within the shell 1, so that the raw water passes as adownflow from upper end portions of the bundled upright long fibers 3toward their lower end portions. Suspended solids are thus trapped ininterstices 40 between the bundled long fibers 3, whereby filtered wateris withdrawn through a filtered water outlet line 6.

Since the bundled long fibers 3 are somewhat bent or collapsed at lowerparts thereof by the stream of the downflow, the lower parts of the longfibers 3 are packed at a somewhat higher density than their upper partswhich extend vertically. As a result, no channeling of the raw wateroccurs so that the solids suspended in the raw water can be effectivelytrapped by the bundled long fibers. Where the solids suspended in theraw water are fine solid particles, a coagulant such as polyaluminumchloride, aluminum sulfate or a cationic, anionic or nonionic highmolecular-weight material may be added as a filter aid to the raw water.

As the feeding of the raw water is continued, more and more suspendedsolids are trapped as they are and/or as coagulated coarser solids ininterstices between the long fibers in the bundles so that the pressuredrop due to filtration resistance increases gradually. Along with theincrease of the pressure drop, the extent of bending or collapse of thebundled long fibers at the lower parts thereof increases so that thevertical length of the bundled long fibers decreases little by little asdepicted in FIG. 2. However, this bending or collapse does not advanceto upper end portions of the bundled long fibers, and even at the end ofthe feeding of the raw water the upper end portions of the bundled longfibers are still extending vertically.

The feeding of the raw water is continued in the above manner until thepressure drop reaches a predetermined value or the quality of thefiltered water falls to a prescribed level. Backwashing is then carriedout in the following manner.

The valves 9,13 are closed, and the valves 10,12 are opened to allowcompressed air to flow in through the air feed line 8.

By the inflow of the compressed air, the water inside the shell 1 isagitated and at the same time the bundled long fibers are shaken. Theinterstices which have been defined between the fibers in the bundlesare thus broken up, so that the solids and/or solid aggregates trappedin the interstices are disintegrated and the solids and/or solidaggregates deposited on the bundled long fibers are separated.

While continuously feeding the compressed air or after closing the valve12 to stop the inflow of the compressed air, the valve 11 is opened withthe valve 10 maintained open. This allows backwash water to flow intothe shell 1 via the backwash water feed line 7.

Since the bundled long fibers 3 are fixed at the lower end portionsthereof on the perforated plate 2 but are free-standing at the upper endportions thereof, the upflow of the backwash water flowed into the shell1 causes the bundled long fibers 3 to extend as a streamer and also toshake with the lower end portions thereof fixed on the perforated plate2 as illustrated in FIG. 3.

The trapped solids are hence no longer allowed to remain among and onthe bundled long fibers. The solids are hence released from the bundledlong fibers and a backwash effluent containing a great deal of solidssuspended therein is discharged through the backwash water dischargeline 5.

In addition to the above-mentioned backwashing method, the backwashingof the column filter according to the first embodiment of this inventioncan also be performed by feeding compressed air intermittently whilefeeding backwash water as an upflow at a constant rate. In fact, thisbackwashing method is more effective in many instances.

Because the lower end portions of the bundled long fibers 3 are fixed onthe perforated plate in the column filter according to the firstembodiment of this invention, the bundled long fibers 3 are not washedout of the shell 1 even when backwash water and/or compressed air arefed at a high flow velocity.

Compared to conventional filters using a filter medium such as sand oranthracite, the column filter according to the first embodiment of thisinvention allows to feed backwash water and/or compressed air at ahigher flow velocity. Further, absolutely no flow obstacle such as ascreen is provided in a top part of the shell. Therefore, the columnfilter according to the first embodiment of this invention can flush outtrapped solids in a shorter time and effectively.

Where the upright cylindrical shell 1 has a relatively large diameter,it is desirable to provide one or more upright partition wallssubstantially in parallel with the bundled long fibers 3 within theshell 1 to divide the interior of the shell 1 into vertically-extendingsections so that the bundled long fibers 3 may be prevented from beingbent horizontally or obliquely during the feeding of the raw water.

A description will next be made of the bundled long fibers 3 useful inthe practice of this invention.

As already described above, the bundled long fibers 3 useful in thisinvention are somewhat bent or collapsed at lower parts thereof and arereduced in height at an initial stage even when a raw water of arelatively high flow velocity is fed as a downflow, and uponcontinuation of the feeding of the raw water, the lower parts of thebundled longer fibers 3 are progressively bent further and their heightis reduced little by little. The bundled long fibers 3 are thus requiredto have stiffness and packing quantity or density sufficient to preventtheir horizontal bending and to allow them to remain upright as a wholein the upright cylindrical shell 1 during the feeding of the raw water.Usable fibers include synthetic fibers such as acrylic fibers, polyesterfibers and polyamide fibers as well as natural fibers such as cotton andwool.

As these synthetic fibers or natural fibers, it is preferable to useunits of non-twisted monofilaments having a diameter not greater than 80μm, usually around 35 μm or so.

However, units of twisted filaments may also be used so long as theytake the form of bundled long fibers which are not bent horizontally andremain upright as a whole in the upright cylindrical shell 1 while theraw water is passed.

As the packing density of the long fibers in the bundles employed inthis invention becomes higher, finer suspended solids can be removed toprovide filtered water of higher quality. However, the pressure dropincreases.

On the other hand, the quality of filtered water falls as the packingdensity of the long fibers becomes lower. In contrast, the pressure dropdecreases.

It is accordingly desirable to choose bundles of long fibers having amost suitable packing density depending on the nature or concentrationof solids suspended in the raw water. When bundles of long fiberscomposed of units of non-twisted monofilaments 1,000-3,000 mm long areused by way of example, it is preferable to pack bundles to give apacking density of 25-110 kg in terms of their dry weight per bulkvolume m³ of the bundles.

The length of the bundled long fibers may be limited to the rangeexceeding 1,000 mm--especially longer than 2,000 mm--but shorter than3,000 mm in this invention. This is for the following reasons.

If the length of the bundled long fibers is 1,000 mm or shorter, thefiltration time is so short that the column filter is not satisfactoryas an industrial apparatus. Although the filtration time becomes longeras the length of the bundled long fibers increases, the filtration timereaches to saturation at the length exceeding about 3,000 mm. Then, theadvantages available from the use of bundled fibers having a greaterlength are reduced and the use of bundled fibers of such a greaterlength results merely in a higher manufacturing cost. It is thereforenot preferable to use bundled fibers having a length outside the aboverange. In other words, marked advantages in filtration time can bebrought about where the bundled long fibers have a length in the rangeof 1,000-3,000 mm.

The quality of filtered water is somewhat inferior where the length ofthe bundled long fibers is 1,000 mm or less. There is however nosignificant difference in the filtered water quality among the bundledfibers length range greater than 1,000 mm but shorter than 3,000 mm.

The column filter according to the first embodiment of this invention,in which some modifications have been incorporated, is illustrated inFIG. 4. The perforated plate 2 is arranged transversely in the lowerpart of the upright cylindrical shell 1 . Holders 2' for bundles of longfibers, which will be described subsequently, are attached to theperforated plate 2. The bundled long fibers 3 are fixed at lower endportions thereof by the holders 2' but are free-standing at upper endportions thereof. The bundled long fibers 3 are packed at a relativelyhigh density within the shell 1, so that the bundled long fibers 3extend upright in the shell 1.

The raw water feed line 4 is provided in communication with the top partof the shell 1, and the backwash water discharge line 5 is communicatedas a branch to the raw water feed line 4.

A filtered water outlet line 6 is provided in communication with abottom part of the shell 1. A backwash air feed line 8 is provided as abranch in communication with the water outlet line 6. A backwash waterfeed line 7 is also communicated as a branch to the outlet line 6.

In this modified embodiment, the backwash air feed line 8 may bebifurcated to communicate with the shell 1 at antipodal points on theside wall of the shell 1. A single backwash air feed line may certainlybe used without problems. Numerals 9, 10, 11, 12 and 13 indicate valves,respectively.

A description will next be made of the holders 2' which are used for thebundled long fibers in the column filter of this invention.

The holders 2' constitute the most important feature in this invention.As illustrated in FIGS. 5 and 6 by way of example, each of the holders2' is formed basically of a cap 21 as an upper cap member, said capdefining many circular orifices 20 through its top wall and its sidewall 42, and a tube 23 as a lower hollow member, said tube defining asaxially-elongated openings slits 22 through a side wall 41 thereof andopening at both upper and lower ends thereof.

Attachment of each holders 2' to its corresponding perforation 24 of theperforated plate 2 is performed in the following manner.

First of all, a pair of holes (not shown) are bored in an antipodalrelation through the side wall 41 of the tube 23 at points adjacent theupper end of the tube 23. The T-shaped head of a T-bolt 25 is insertedinto the holes so that the tube 23 is suspended from the T-bolt 25.Incidentally, the outer diameter of the tube 23 is slightly smaller thanthe diameter of the perforation 24 of the perforated plate 2 so that anupper end portion of the tube 23 may be closely fitted in the theperforation 24.

Thereafter, the stem of the T-bolt 25 is inserted upwardly through theperforation 24 until the upper end portion of the tube 23 is fitted inthe perforation 24. A nut 27 with legs 26 is then applied on the T-bolt25 to hold the T-bolt 25 in the perforation 24 by the legs 26.

The cap 21 with the bundled long fibers 3 fixed at their low endportions on the entire peripheral side wall 42 by a band 28 is thenplaced over the perforation 24 with the stem of the T-bolt 25 extendingthrough a hole 29 of the cap 21. A nut 30 is next applied on the T-bolt25 so that the cap 21 is fixed over the perforation 24.

In a manner as described above, the holders 2' for the bundled longfiber are attached to the respective perforations 24 of the perforatedplate 2.

Incidentally, the orifices 20 of the cap 21 permit the passage of fluidssuch as filtered water, compressed air and backwash water therethrough.It should however be noted that the formation of the orifices 20 in thetop wall of the cap 21 is not essential. It is however necessary to formmany orifices through the entire peripheral side wall 42 of the cap 21.The circular orifices 20 may be replaced by slits.

The slit 22 of the tube 23 primarily serves to permits passage ofcompressed air therethrough. The slit 22 may be replaced by a pluralityof small holes arranged vertically.

The structure of the holders 2' for bundled long fibers and the mannerof their attachment shown in FIGS. 5 and 6 are merely illustrative.Their detailed structures may be modified as desired, provided thatdescribed basically, the holders 2' for bundled long fibers, each ofsaid holders 2' being composed of the cap 21 defining many orifices 20or slits through the entire peripheral side wall 41 thereof and the tube23 defining a slit 22 or vertically-arranged small holes through theside wall 41 thereof and opening at both upper and lower ends thereof,are attached to the respective perforations 24 of the perforated plate 2arranged transversely within the shell 1 with the cap 21 being locatedabove the perforated plate 2 and the tube 23 being positioned below theperforated plate 2; and the lower end portions of the bundled longfibers 3 are fixed to lower peripheries of the side walls 42 of theassociated caps 21.

Where the upright cylindrical shell 1 has a relatively large diameter,it is desirable, as shown in FIG. 6, to provide one or more uprightpartition walls 43 substantially in parallel with the bundled longfibers 3 within the shell 1 to divide the interior of the shell 1 intovertically-extending sections so that the bundled long fibers 3 may beprevented from being bent horizontally or obliquely during the feedingof the raw water.

Before the present invention, the fixing of lower end portions ofbundled long fibers was conducted as shown in FIG. 9. Each bundle oflong fibers 3 was folded and bound at the folded part by a ring 31. Thefolded part was positioned to close up each perforation 24 of theperforated plate 2. Using a bolt 32 connected to the ring 31, the bundleof long fibers 3 was fixed with a stem-like holder 33 and a nut 34. Theconventional fixing method was therefore accompanied by drawbacks to bedescribed next.

In such a structure, the long fibers bundled together at the ring 31were arranged very densely and a raw water always had to pass throughthe densely-bundled part of the long fibers during the filtration. Thedensely-bundled part of the long fibers was therefore prone to cloggingwith suspended solids. Once the densely-bundled part was clogged, it wasdifficult to remove the thus-trapped solids even when backwashing wasconducted.

Because, when a fluid such as compressed air or backwash water is causedto pass upwardly through each perforation 24 in such a structure asdepicted in FIG. 9, the fluid passes preferentially through the point ofcontact between the upper surface of the perforated plate 2 and thebundled long fibers 3 and hardly passes through the fiber part throughwhich the fluid is supposed to pass, namely, the densely-bundled part ofthe long fibers. When the nut 34 is tightened firmly to increase thedegree of contact between each bundle of long fibers 3 and the edge ofits corresponding perforation 24, the fluid preferentially passesthrough the perforations 24 having bundles of long fibers 3 less cloggedwith trapped solids. The fluid therefore does not pass through thebundles of long fibers which require washing, namely, through theperforations 24 having bundles of long fibers 3 whose densely-bundledparts are clogged with trapped solids.

When the filter is operated continuously under the above conditions,more solids gradually remain in the densely-bundled part of each bundledlong fibers. This causes the initial pressure drop to increase graduallyin the life of column filter, and in some instances gives seriousproblems to the filtration plant itself.

As another drawback, even when the bundle of long fibers 3 is so fixedas to squeeze itself in the corresponding perforation 24, a smallclearance still remains between the peripheral edge of the perforation24 and the bundles of long fibers 3. An increased pressure drop causessuspended solids to leak through the clearance, so that the quality ofthe filtered water falls toward the end of filtration.

In a column filter operation of the present invention, as the filtrationproceeds and more solids are trapped in the interstices among the longfibers 3 in the bundles, the pressure drop increases gradually. As thepressure drop increases, the vertically-extending long fibers in thebundles begin to be bent from the lower end portions thereof so thattheir vertical length decreases gradually.

When the increase of the pressure drop has reached a predeterminedvalue, backwashing is conducted in the following manner.

Referring again FIGS. 4 to 6, the valves 9,13 are closed to stop thefiltration and the valves 10,12 are then opened to feed compressed airthrough the backwash air feed line 8. Since the bottom part underneaththe perforated plate 2 is filled with the filtered water, the inflow ofthe compressed air firstly causes the filtered water to flow out throughthe individual orifices 20 of each cap 21. However, a water level L issoon formed below the perforated plate 2 as depicted in FIG. 5. A layerA of the compressed air is therefore formed above the water level L. Thecompressed air therefore enters the tube 23 through a portion of theslit 22, said portion being located above the water level L, and thenblows out through the individual orifices 20 of each cap 21.

By the blow-out of the compressed air, the water inside the uprightcylindrical shell 1 is agitated and the bundled long fibers 3 areshaken. The interstices which have been formed among the fibers 3 arehence broken up to disintegrate solids trapped and accumulated therein.In addition, solids deposited on the bundled long fibers 3 are alsoshaken off.

In particular, the water which is contained underneath the perforatedplate 2 at the beginning of feeding of the compressed air is lifted atonce by the compressed air and jets out as high-velocity plug flowsthrough the individual orifices 20 of each cap 21, followed by theflow-out of the compressed air. The densely-bundled part of the bundledlong fibers 3 located around each cap 21 can therefore be washedeffectively by the passage of the fluids therethrough.

While continuing the above-described inflow of the compressed air orafter closing the valve 12 to stop the feeding of the compressed air,the valve 11 is opened to feed backwash water through the backwash waterfeed line 7.

The backwash water thus fed enters primarily through the lower openingof each tube 23 and flows out through the individual orifices 20 of eachcap 21.

Since the bundled long fibers 3 are fixed at the lower end portionsthereof and are free-standing at the upper nd portions thereof, thebundled long fibers 3 are caused to extend and are shaken like astreamer by the upflow of the backwash water.

The solids released from the bundled long fibers 3 by the impact of theinflow of the compressed air are hence flushed by the backwash water, sothat the backwash water containing a great deal of solids suspendedtherein is discharged through the backwash water discharge line 5.

Different from the backwashing method described above, the backwashingcan also be performed by feeding compressed air intermittently throughthe backwash air feed line 8 while feeding backwash water at a constantrate through the backwash water feed line 7.

In this case, high-velocity plug flows are formed as described abovewhenever the compressed air is fed so that more effective backwashing isfeasible.

Another embodiment of the supports for bundles of long fibers, which areemployed in this invention, is illustrated in FIG. 7. A conicaldiaphragm seat 37 defining many through-perforations 36 is provided inan upper interior part of each cap 21'. Provided inside the diaphragmseat 37 is a conical diaphragm 38 which can close up thethrough-perforations 36 when brought into close contact with the innersurface of the diaphragm seat 37. Slits 20' are formed through a topwall of the cap 21' and also through its side wall 42'. Through a sidewall 41' of a tube 23', a plurality of small slit 22' are spacedlyformed in an axial, namely, vertical row. The remaining structure issimilar to the holder shown in FIG. 5. An upper end portion of thediaphragm 38 is secured on the inner surface of the top wall of the cap21, while its lower end portion flares out in the form of a horn. It ismade of a flexible material.

The diaphragm seat 37 and diaphragm 38 function as follows.

During the filtration, the filtered water passes downwardly through theslits 20' and the diaphragm 38 is pushed inwardly by the filtered water.Therefore, the diaphragm seat 37 and diaphragm 38 neither exhibit anyfunction nor interfere with the filtration.

However, they function in the following manner during backwashing.

By an upflow of compressed air or backwash water fed through the tube23', the diaphragm 38 is outwardly pushed and opened so that thediaphragm 38 is brought into close contact with the inner surface of thediaphragm seat 37. As a result, the compressed air or backwash water isallowed to pass only through the slits 20'A formed below the diaphragmseat 37.

Accordingly, the compressed air or backwash water is allowed to flow outpreferentially through the lower end portion of the fixed part of thebundled long fibers 3, so that the hardly-washable lower end portions ofthe bundled long fibers 3 can be washed effectively. The construction ofFIG. 7 is particularly effective for ensuring the flushing of trappedsolids from the bundled long fibers during backwashing.

To demonstrate the advantages of the present invention, the presentinvention will hereinafter be described specifically by the followingexample. Example:

A perforated plate was arranged in a lower part of an uprightcylindrical shell made of stainless steel. At a straight wall portion ofthe shell, the diameter and height were 650 mm and 4,000 mm,respectively. Seven bundles of long fibers composed of units ofnon-twisted acrylic monofilaments having a diameter of 35 μm, whoselengths ranged from 800 mm to 3,500 mm, were separately fixed at lowerend portions thereof but left free-standing at upper end portionsthereof, so that filters as illustrated in FIGS. 4 to 6 wereconstructed, respectively.

The end point of feeding of a raw water was set at 0.5 kg/cm² in termsof pressure drop (initial pressure drop: 0.1 kg/cm²). After the end ofthe feeding of the raw water through each filter, the filter wasbackwashed first with air having a linear upflow velocity of 1,000 Nm³/m² for 1 minute, with 1,000 Nm³ /m² of air and 100 m/hr of backwashwater for 3 minutes, and then with 100 m/hr of backwash water alone for1 minute. The pressure drop through the packed medium layer was reducedto the initial pressure drop, thereby indicating that the solids trappedby the bundled long fibers had been flushed completely and the theconditions before the filtration had been restored.

The above experimental filtration was conducted for 6 months. Thefiltration times and the average turbidities of treated waters for therespective lengths of the bundled long fibers were as shown in thefollowing table.

    ______________________________________                                        Length of bundled                                                                           Filtration                                                                             Average turbidity of                                   long fiber, m time, hr treated water, degree                                  ______________________________________                                        0.8           4        11                                                     1.0           5.3      8                                                      1.5           8        7                                                      2.0           10       7                                                      2.5           12       7                                                      3.0           13       7                                                      3.5           13.5     7                                                      ______________________________________                                    

We claim:
 1. A column filter using bundles of long fibers, said filterbeing provided with an upright cylindrical shell and the bundles of longfibers being arranged substantially upright inside the shell with lowerend portions thereof being fixed and upper end portions thereof beingfree-standing, whereby, when a raw liquid with suspended solids thereinis fed to a top part of the shell and is then allowed to pass from theupper end portions of the bundles of long fibers toward their lower endportions, the suspended solids are at least partly trapped ininterstices among the, fibers, the improvement comprising: structurehaving properties for providing an operable filtration time longer thana column filter in which the bundles of long fibers have a length ofless than 1000 mm, including, a supporting member is arrangedtransversely in a lower interior part of the shell, the bundles of longfibers have a length in a range longer than 1,000 mm but shorter than3,000 mm and are fixed at the lower end portions thereof on an uppersurface of said supporting member, and the packing density of thebundles is in a range such that the bundles may retain 30-80% of theoriginal height thereof while the raw liquid with suspended solids isbeing fed into the shell and compressing the bundles toward thesupporting member.
 2. The column filter as claimed in claim 1, whereinthe packing density of the bundles is in a range of 25-110 kg in termsof their dry weihgt per bulk volume m³ of the bundles.
 3. The columnfilter as claimed in claim 1, wherein the fibers in the bundles have adiameter up to 80 μm.